docs: def purview_state(self, direction):
          """The state of the purview when we are computing coefficients in
          ``direction``.
  
          For example, if we are computing the cause coefficient of a mechanism
          in ``after_state``, the direction is``CAUSE`` and the ``purview_state``
          is ``before_state``.
          """
          return {
              Direction.CAUSE: self.before_state,
              Direction.EFFECT: self.after_state
          }[direction]